1. Field of the Invention
The present invention relates to a method of forming image segments in a lithographic process and, in particular, to a method of preparing a mask for use in x-ray lithography.
2. Description of Related Art
As device dimensions continue to shrink, advanced lithographic methods are being developed to replace conventional optical lithography. Robust and effective etch processes are required to pattern submicron ( less than 0.1 xcexcm width) features on masks used for advanced lithography. This is particularly important on masks used for 1xc3x97 x-ray lithography since there is no demagnification of the mask during exposure. Of paramount importance is the selection of a refractory film stack (absorber and hardmask layers) that satisfies the stringent film stress requirements and has good etching characteristics. The absorbing layer is typically an amorphous material composed of a refractory metal, such as tantalum or tungsten. The absorbing layer could also include portions of silicon, geranium or boron, and possibly nitrogen which is incorporated to ensure an amorphous structure. The hardmask layer is typically a chromium, chromium nitride, or silicon oxynitride (SiON) film, but may also be any other film which has good stress characteristics and high etch selectivity to the absorber layer.
Masks are typically fabricated by patterning the hardmask layer from resist images with a plasma etch followed by a second high density plasma etch to define the absorber images. Since x-ray masks consist of membranes formed in the central area of wafers, the actual patterned area represents a smaller fraction of the actual wafer surface area; as little as 10% of the wafer surface area in some cases. If a certain mask pattern with 20% coverage was exposed, it would result in 2% total pattern density on the x-ray mask. Refractory metal etches are very challenging when the mask pattern density is this small. Very aggressive etches are required to pattern sub-0.25 xcexcm features on low pattern density masks, and even in these cases, the resulting image quality can be poor.
Typically in the prior art, a tantalum silicon etch utilizes high power on the order of 70 to 100 watts at higher temperatures of 50-70xc2x0 C. Such etch has typically low selectivity to SiON on the order of less than four to one. Image size is generally difficult to achieve for images less than 180 nm and is typically actually 130 to 160 nm for 180 nm nominal images. Additionally, vertical side walls are difficult for images less than 180 nm and there is usually a lower resolution for some optical endpoint systems.
A typical prior art process for producing a dark field x-ray mask is depicted in FIGS. 1 and 2. In FIG. 1 a substrate 20 made of silicon or the like has layered on the upper and lower surfaces a membrane layer 22 of silicon carbide, boron-doped silicon, diamond film or the like. The wafer substrate 20 has an etched or cut out region 21 on its lower surface to expose the lower surface of upper membrane layer 22. The region on the wafer upper surface above the etched out area 21 is referred to as active area 60, while the region on the wafer upper surface outside the active area above the substrate thickness is referred to as inactive area 70. The mask is formed in the active area of the substrate and lithographic exposure is carried out through the active area. The wafer is mounted on a pyrex ring 30 for handling purposes.
Above the upper membrane layer 22 there is applied, in sequence and directly over each lower layer, an optional etch stop layer 24 of chromium, an x-ray absorber layer 26 typically of tantalum silicon (TaSi) or other absorber, a hardmask layer 28 typically of SiON (or Cr) and a resist layer 32. Resist layer 32 is shown having openings 34 created by electron beam exposure, baking, developing and removal of these regions which will eventually form the mask. These layers are usually applied before the lower surface is etched out.
After first etching the hardmask layer 28 and then the absorber layer 26, FIG. 2 shows the resultant mask with the resist layer and hardmask layer stripped by conventional means. In the case of a dark field mask, there is left an x-ray mask having the absorber layer 26 completely covering the inactive area 70 and covering only selected portions of the active area 60.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved method of forming image segments for lithographic masks, and in particular for low pattern density x-ray lithography masks or other advanced lithography such as SCALPEL (Scattering with Angular Limitation Projection E-beam Lithography) or EUV (Extreme Ultra-violet Lithography).
It is another object of the present invention to provide a method of preparing lithography image segments and masks which utilize lower power and lower temperature in the etching of the absorber or similar layer on the mask.
It is a further object of the invention to provide an improved method of forming image segments and lithography masks which may accommodate smaller image sizes and provide more vertically etched sidewalls.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a method of forming image segments comprising providing a workpiece having active and inactive surface regions; applying a photoresist over the workpiece covering both active and inactive regions and exposing and developing at least the active region to create mask pattern images. The method then comprises removing photoresist from at least a portion of the inactive region and removing portions of the workpiece using the remaining photoresist as a mask to form image segments in the active region.
The photoresist may be a positive or negative photoresist and the image segments may be used to form a dark field x-ray mask. The mask pattern images of the active region may be patterned by electron beam or UV irradiation. The method may include blocking the active region, for example, by masking, while exposing and developing the inactive region with electron beam irradiation or UV light to remove photoresist.
The method may include applying an absorber, for example, an x-ray absorber, prior to applying the resist and using the remaining photoresist as a mask to leave portions of the absorber on the workpiece to form a lithographic mask. A hardmask layer may be applied to the workpiece prior to the photoresist and the method then includes the step of removing portions of the hardmask layer using the remaining photoresist as a mask.
In another aspect, the present invention provides a method of preparing an x-ray mask comprising providing a substrate, and applying sequentially to a surface of the substrate i) an etch stop layer resistant to etchant for an x-ray absorber, and ii) an x-ray absorber layer. The method then includes removing a portion of the substrate below the layers to create an active region of the substrate above the removed portion of the substrate and an inactive region over remaining portions of the substrate, applying a resist layer above the absorber layer, and exposing a portion of the resist layer using electron beam irradiation and developing the resist layer to form a latent mask image over the active region of the substrate. The method then includes removing the exposed portion of the resist layer to leave a resist layer mask image over the active region of the substrate, and etching the absorber layer to leave an absorber layer mask image over the designated active region of the substrate while simultaneously removing the absorber layer from the inactive region to increase the effective pattern density during etching.
The method optionally includes applying a hardmask layer over the x-ray absorber layer and etching the hardmask layer to leave a hardmask layer mask image over the active region of the substrate while separately removing the hardmask layer from the inactive region. The hardmask layer may also be applied only over the active region of the substrate by masking the inactive region of the substrate to prevent application of the hardmask layer.
Preferably, the resist is a positive photoresist, and the method further includes exposing the inactive region with electron beam irradiation or UV light. The substrate may include a membrane layer on the surface wherein the membrane layer is not removed during removal of the substrate to create the active region.
Etchant byproducts used during etching of the absorber layer may be monitored for presence of absorber material used in the absorber layer to determine an endpoint for the etching. Additional absorber may be provided adjacent the substrate and the additional absorber may be etched while etching the absorber on the substrate.